Embodiments of the invention relate generally to electric drive systems including hybrid and electric vehicles and, more particularly, to charging energy storage devices of an electric vehicle using a multiport energy management system.
Hybrid electric vehicles may combine an internal combustion engine and an electric motor powered by an energy storage device, such as a traction battery, to propel the vehicle. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while internal combustion engines (ICEs) may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in hybrid vehicles to be smaller and more fuel efficient.
Purely electric vehicles use stored electrical energy to power an electric motor, which propels the vehicle and may also operate auxiliary drives. Purely electric vehicles may use one or more sources of stored electrical energy. For example, a first source of stored electrical energy may be used to provide longer-lasting energy, such as a low-voltage battery (commonly referred to as an ‘energy battery’) while a second source of stored electrical energy may be used to provide higher-power energy for, for example, vehicle acceleration, using a high-voltage battery (commonly referred to as a ‘power battery’). Known energy storage devices may also include an ultracapacitor, which tends to have fast charging and discharging capability and provides long life operation.
Plug-in electric vehicles, whether of the hybrid electric type or of the purely electric type, are typically configured to use electrical energy from an external source to recharge the energy storage devices. Such vehicles may include on-road and off-road vehicles, golf carts, neighborhood electric vehicles, forklifts, and utility trucks as examples. Known charging devices include a multiport energy storage management system (ESMS) for charging both low voltage and high voltage energy storage systems of an electric vehicle. Typically, an ESMS includes buck-boost converters which can be used in conjunction with one another in order to flexibly apply charging voltages to a variety of devices having different charging voltage requirements. An ESMS also typically includes a high voltage side and a low voltage side. In one known ESMS device having four ports, two of the ports are on a high voltage side of the device and two of the ports are on a low voltage side of the device. The high voltage side is typically used for charging from a utility grid or renewable energy source (one port on the high voltage side) and for providing charging power to a power battery (another port on the high voltage side). The low voltage side is typically used for charging low voltage devices such as energy batteries and ultracapacitors of the electric vehicle (ports on the low voltage side) and may, in some embodiments, also include adaptability to a low voltage charging source as well, in one of the low voltage ports.
A power battery, incidentally, is typically included in order to provide high power bursts for acceleration of the vehicle, as opposed to an energy battery, which is typically included in order to provide long-range cruising energy to the vehicle and it is therefore desirable to operate as a high voltage device. Thus, because of the high power requirements of the power battery, high voltage energy storage devices such as power batteries typically operate under a high voltage operation of 400 V or more, while low voltage energy storage devices such as energy batteries typically provide high energy storage and operate at a much lower nominal voltage, such as 120 V or below. Ultracapacitors can be used in either high or low voltage applications and thus can be included on either the high side or the low side of the ESMS charging device, depending on their type of use (high bursts of power vs. energy storage for cruising).
Because of the buck-boost converters in the ESMS, multiple arrangements of energy storage devices and power sources may be utilized in order to charge the energy storage devices. That is, a known ESMS is flexibly configurable in that a charging voltage may be first bucked down, and then boosted up to a desired charging voltage on the high voltage side. And, because of the bucking and subsequent boosting operations, the charging on the high side may be either above or below the charging voltage provided externally. Similarly, the charging voltage may be bucked to the lower voltage of the low voltage side as well. Further, because of the multiple buck-boost converters in an EMS, the charging voltage may be simultaneously provided to charge both the high voltage device on the high side, as well as one or more low voltage devices on the low side. That is, a single high voltage supply may be split to simultaneously provide energy to the high side and the low side devices, or to two low side devices, as examples.
Known devices that split power for charging multiple energy storage devices are typically optimized based simply on a condition of the devices that are being charged. That is, known charging or ESMS devices typically base their power split on factors such as the state-of-charge of the device(s) and/or the voltage at each respective charging port. Although such an optimization often can be adequate to provide a maximum overall rate of charging to the combination of devices being charged, such a charging scheme does not take into account additional factors such as the overall implications to the life of the devices themselves that are being charged, their temperature limits, and the like. That is, although energy storage devices may be physically capable of receiving a high rate of charge in order to minimize charging time of all devices, it may not be desirable to do so if the long-term cost to one or more of the devices is a drop in life.
In other words, the lifecycle cost and eventual need to replace storage devices such as power batteries, energy batteries, and ultracapacitors may not be worth the marginal decrease in charging time when charging is based on a state-of-charge alone. In fact, because known charging devices determine power splits and charging rates without taking into account the specifics of the devices themselves (but rather are simply based on a state-of-charge or a voltage at the charging terminals), the devices not only have a longtime risk of life, but are also at risk of catastrophic failure if charged beyond a rate than the device can handle.
It would therefore be desirable to provide an apparatus and control scheme to optimize overall recharge time for multiple energy storage devices of an EV while taking into account the life implications of the charging scheme.